Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ an induction heater to warm up the fixing device quickly to a predetermined fixing temperature with reduced energy consumption. FIG. 1 illustrates a fixing device 19 with such induction heater 24 installed. As shown in FIG. 1, the induction heater 24 generates a magnetic flux that generates an eddy current in proximity to a heating roller 23. The eddy current causes the heating roller 23 including a heat generation layer to generate Joule heat by electric resistance of the heating roller 23, which in turn heats a fixing belt 22 stretched over the heating roller 23 and a fixing roller 21 and rotating in a predetermined direction of rotation. As a recording medium P bearing a toner image T passes through a fixing nip N formed between a pressing roller 34 and the fixing belt 22, the heated fixing belt 22 and the pressing roller 34 apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P.
The induction heater 24 includes an exciting coil 25 that generates a magnetic flux and a plurality of arch cores 26c that guides the magnetic flux generated by the exciting coil 25 to the heating roller 23. In order to minimize leakage of magnetic flux, the induction heater 24 further includes a center core 26a disposed at a center of the induction heater 24, a side core 26b1 disposed at one end of the induction heater 24, and a side core 26b2 disposed at another end of the induction heater 24 in the direction of rotation of the fixing belt 22. For example, each arch core 26c has a planar edge face 26c1 disposed at one end of the arch core 26c in the direction of rotation of the fixing belt 22 and contacting a planar face of the side core 26b1 and a planar edge face 26c2 disposed at another end of the arch core 26c in the direction of rotation of the fixing belt 22 and contacting a planar face of the side core 26b2. Thus, the plurality of arch cores 26c and the side cores 26b1 and 26b2 contacting the arch cores 26c can direct the magnetic flux toward the heating roller 23 while minimizing the leakage of magnetic flux from between the arch cores 26c and the side cores 26b1 and 26b2.
Generally, a ferrite core, which is produced by compression-molding and sintering ferrite powder, is used in the induction heater 24. During the sintering process, the core shrinks. Taking the arch core 26c for instance, a shrinkage of an edge portion in proximity to the span of the arch core 26c may differ from a shrinkage of other portions of the arch core 26c, widening the span to a degree varying from one arch core 26c to the next. Accordingly, some arch cores 26c may contact the planar faces of the side cores 26b1 and 26b2 flush, that is, evenly and precisely over the entire planar edge faces 26c1 and 26c2 of the arch core 26c, while other arch cores 26c may contact the planar faces of the side cores 26b1 and 26b2 unevenly, at only a part of the planar edge faces 26c1 and 26c2 of the arch core 26c. Such variation in contact state in which the arch cores 26c contact the side cores 26b1 and 26b2 may affect generation of a magnetic path in the longitudinal direction of the induction heater 24 parallel to the long axis of the heating roller 23. For example, some sections of the induction heater 24 where the magnetic path is appropriately generated heat the heating roller 23 efficiently. Conversely, other sections of the induction heater 24 where the magnetic path is inappropriately generated leak the magnetic flux generated by the exciting coil 25 from between the arch core 26c and the side cores 26b1 and 26b2, degrading heating efficiency of the heating roller 23 and thus resulting in variation in the temperature of the heating roller 23 in the axial direction thereof. Consequently, the heating roller 23 does not heat the fixing belt 22 uniformly in the axial direction thereof, thus forming a faulty toner image T on a recording medium P.
To address these problems, additional cutting is required to cause the planar edge faces 26c1 and 26c2 of the arch core 26c to precisely contact the planar faces of the side cores 26b1 and 26b2. However, such additional cutting increases manufacturing costs of the arch cores 26c. 